Harrison, 20, has become Chris Powell's first signing as head coach of the Blues.
The forward scored once in 15 appearances during a loan spell at Yeovil Town last season, and will now spend the rest of the 2017-18 campaign with League One outfit Southend.
"I'm buzzing to be here. When I found out Southend were interested in me I thought it was perfect move for me," Harrison told the club's official website.
"I've spoken to the manager and he's really keen on me being here and made me feel really welcome. People at Spurs have spoken to me about the manager and how he's developed young players so I'm really looking forward to working with him."
"League Two was what I expected in the physicality side of it. It was much different to U23's football. I'm looking for a new challenge in League One and I'm ready to make that jump.
"I was injured at the beginning of the season but since I've been back I've played quite a few games in the U23 side at Spurs. But now I just want to play men's football regularly and show what I'm about.
Harrison, who could make his debut against Peterborough United at the weekend, has scored three times in eight Premier League 2 appearances for Tottenham's Under-23 side this season.